dpdk: integrate with Sphinx based docs and manifest file

- Update build script:
  * Remove 'git clone' as repos are managed through manifest file
  * Update directory structure to match the one in manifest file
- Replace quickstart.md with doc/quickstart.rst
- Update DPDK L3 Fwd application in user guide
- Update README.md
  * Remove reference to specific solutions releases
  * To point to quickstart and user guides

Signed-off-by: Dharmik Thakkar <dharmik.thakkar@arm.com>
Signed-off-by: Honnappa Nagarahalli <honnappa.nagarahalli@arm.com>
Signed-off-by: Lijian Zhang <Lijian.Zhang@arm.com>
Reviewed-by: Nathan Brown <Nathan.Brown@arm.com>
Reviewed-by: Tianyu Li <Tianyu.Li@arm.com>

Change-Id: Iee9d3b1c9fe8ae1d49aeed8d36173d74bdbf3266